Several forms of photovoltaic cells have been developed for converting solar energy to electrical energy; however, the efficiency of known systems is low and, when efficiencies are improved through the use of more efficient converters, the cost of the converters is high. It has been proposed to decrease the conversion cost and to increase the conversion efficiency by concentrating the solar energy, through the use of optical systems, onto the converters. With such systems the efficiency of conversion can be further increased. Cost consideration on each of the several components of a conversion system indicates that, with light concentrators, more expensive photovoltaic converters may be employed; however, there are economic cost limits on the concentrator system. Further, as the concentration of light onto the converter becomes more intense, there is a need for dissipating the heat derived from the concentrated light because the efficiency of some converters drops as the heat of the converter increases.
From a study of the above recited considerations, it can be shown that, with the use of concentrator systems that can decrease the cost of energy conversion by increasing conversion efficiency, the emphasis can shift from converter cell cost to cell efficiency. Thus, if the cell efficiency can be made great enough, a concentrator system can produce electricity more cheaply than the same area of a lower cost array.
These observations lead to the consideration of high efficiency stacked multijunction solar cells with each cell responsive to a different energy band of solar energy and with a concentrator for concentrating the energy and tracking the cell toward the source of energy. However, a key requirement for the successful operation of a stacked multijunction solar cell is the requirement that the stacked junction be series-interconnected through low resistance interfaces thereby allowing the flow of light generated current from one junction to the next.